1. Field of the Invention
The present invention relates to a bicycle frame assembly with a main frame section and a shock absorbing system for absorbing shocks transmitted from a rear wheel to the main frame section.
2. Description of the Prior Art
This type of bicycle frame assembly is disclosed, for example, in Japanese Patent Unexamined Publication No. 51-31,437 and Japanese Utility Model Unexamined Publication Nos. 53-101,650 and 54-156,352. The prior art frame assembly known from these publications includes a main frame section which is generally formed of a head tube, a top tube, a down tube and a seat tube. The frame assembly further includes a pair of chain stays and a pair of seat stays, wherein the chain stays are pivotally connected to the main frame section adjacent to the bottom bracket at the lower end of the seat tube. On each side of the rear wheel, the chain stay at its rear end and the seat stay at its lower end are integrally and rigidly connected with each other to form a rear frame section. A shock absorbing device is connected between the upper end of the seat stay and the main frame section.
The rear frame section of the aforementioned known frame assembly is positively secured by a pivotal connection to the main frame section only adjacent to the bottom bracket, so that the frame assembly as a whole lacks in sufficient transverse rigidity. As known in the art, a bicycle frame assembly with a poor rigidity undergoes increased bending and/or twisting deformation in actual use condition of the bicycle. This results in serious disadvantages, such as increased power loss in term of the pedal depression force and increased rolling resistance of tires, due to dislocation of front and rear wheels out of a common plane, as well as enhanced difficulties in steering the front wheel by operating a handle bar, and also in maintaining a stabilized driving posture of the cyclist-bicycle system, etc.
Moreover, conventional shock absorbing devices used for the rear wheel shock absorbing system of a bicycle frame assembly encounter various problems.
For example, coil spring has been widely used as the shock absorbing device, which does not essentially achieve a damping effect by itself and tends to deteriorate a stable riding comfort due to a poor convergence of input vibration. The coil spring is generally designed to have a relatively high fixed spring constant which is primarily determined to achieve an intended shock absorbing function when applied with a heavy load, e.g. during driving on a rough road or off-road land surface. This means that the coil spring cannot absorb light shocks arising from a small amplitude vibration during a normal driving on a relatively smooth road surface. Further, the fixed spring constant of the coil spring makes it difficult to optimize the shock absorbing system in terms of cyclist's weight and posture and/or running surface condition which changes from time to time.
An alternate type of the rear wheel shock absorbing system for a bicycle frame assembly employs a shock absorber which is similar to those used for motorcycles. The shock absorber has a basic arrangement wherein a piston and a spring are combined with a cylinder, and allows the spring constant and damping characteristics to be determined within a wide range. However, due to the above-mentioned basic arrangement, the shock absorber not only requires maintenance, but also is generally complex in structure, heavy in weight and costly. Moreover, this this of shock absorber does not always provide a sufficient piston stroke per unit volume, as required particularly for heavy duty bicycles.